Linearized switch-based power amplifier

ABSTRACT

The invention relates to a power amplifier comprising a level-discretization modulator ( 210 ) and a mixing and amplifying stage ( 225 ), in combination with a properly designed negative feedback path. The feedback path forms an error compensation loop together with a properly selected forward path. The forward path includes at least the mixing and amplifying stage ( 225 ), which provides frequency up-conversion to the radio frequency band and switch-based amplification. The negative feedback path includes a down-conversion mixer ( 280 ) for down-conversion to the initial frequency band, and compensates for distortion caused by components of the forward path. The negative feedback action will generally reduce both distortion caused by non-linear steady state impedance of the switches as well as distortion caused by non-linear steady state impedance of the switches as well as distortion resulting from switching glitches. Preferably, the level-discretization modulator ( 210 ) used by the power amplifier is also included in the forward path of the loop so that the feedback path compensates for distortion, such as quantization noise, caused by the modulator.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention generally relates to the field of poweramplifiers, and more particularly to linearization techniques for poweramplifiers.

BACKGROUND OF THE INVENTION

[0002] In cellular base stations, satellite communication systems aswell as other communication and broadcasting systems of today, it isoften desirable to amplify multiple radio frequency (RF) channelssimultaneously in the same power amplifier instead of using a dedicatedpower amplifier for each channel. However, when using one and the samepower amplifier for the simultaneous amplification of multiple FDMA/TDMA(Frequency Division Multiple Access/Time Division Multiple Access) orCDMA (Code Division Multiple Access) channels or when using multi-levellinear modulation formats, a high degree of linearity is required sothat the phases and amplitudes of all the signal components arepreserved in the amplification process.

[0003] If the linearity is inadequate, the simultaneously amplifiedchannels cross-modulate, causing interference in these and otherchannels. The non-linearities manifest themselves as cross-modulation ofdifferent components of the signal, leading to leakage of signal energyto undesired channels. In addition, the spectra of the signal componentsare normally broadened, causing additional interference within thechannels or in other channels. In the case of multi-level linearmodulation such as QAM (Quadrature Amplitude Modulation), thenon-linearities will normally lead to distortion of the constellationdiagram.

[0004] The two most important properties of a power amplifier areefficiency and linearity. Consequently, the problem of enhancing thelinearity must be solved while preserving high amplifier efficiency. Dueto the amplitude statistics of complex multi-channel or multi-levelsignals, the efficiency must be kept high at all amplitude levels inorder to provide high average efficiency. An additional complication isthat the signals often have wide bandwidth, or wide combined bandwidth,which means that the fluctuations between high and low amplitude levelsare fast.

[0005] A solution to the combined problem of enhancing linearity andpreserving high efficiency has been disclosed by the Applicant in theInternational Patent Application WO 98/11683 and the corresponding U.S.Pat. No. 6,094,458. The proposed power amplifier uses switchingtechniques for the power amplification, and is based on a sigma-deltamodulator in combination with a mixing and amplifying stage. Thesigma-delta modulator includes a quantizer for generating an outputsignal with a finite number of discrete levels. The fixing andamplifying stage includes a plurality of power switches, the voltages ofwhich are proportional to the levels of the quantizer, and the outputsignal of the sigma-delta modulator more or less directly controls thepower switches via a decoder so as to provide appropriate poweramplification. Although the proposed switch-based power amplifier hasvery high efficiency and can benefit from the high linearity of thesigma-delta modulator, there is still a general demand for poweramplifiers with even better linearity characteristics.

[0006] U.S. Pat. No. 5,736,906 relates to a power supply modulatorcircuit for a transmitter. The power modulator circuit is a basebandamplifier with apportioned input impedance, buffered switching of powermodules and linearizing feedback.

[0007] The International Patent Application WO 98/44626 relates to apulse modulation power amplifier for the audio frequency range. Thepower amplifier comprises a pulse modulator, a power amplifier stage foramplifying the modulated signal, the output of which is low-passfiltered to obtain an analog output. The circuit further comprises anegative feedback from the power amplifier stage output to one orseveral loops feeding into one or several pre-amplifier stages precedingthe modulator.

SUMMARY OF THE INVENTION

[0008] It is a general object of the present invention to provide animproved linearization technique for reducing distortion andnon-linearities in a switch-based power amplifier.

[0009] It is a particular object of the invention to provide a highlylinear power amplifier with excellent stability and with reasonablerequirements on the involved power amplifier components.

[0010] A special object of the invention is to improve the linearizationof an amplifier circuit for generating power amplified radio frequencysignals, while maintaining the possibility to operate thelevel-discretization modulator of the amplifier on relatively low inputsignal frequencies.

[0011] It is also an object of the invention to provide a transmitterwith excellent power amplifier efficiency and linearity.

[0012] These and other objects are met by the invention as defined bythe accompanying patent claims.

[0013] A first aspect of the invention concerns a power amplifiercomprising a level-discrezation modulator and a mixing and amplifyingstage, in combination with a properly designed negative feedback path.The feedback path forms an error compensation loop together with aproperly selected forward path. The forward path includes at least themixing and amplifying stage, which provides frequency up-conversion tothe radio frequency band and switch-based amplification. The negativefeedback path includes a down-conversion mixer for down-conversion tothe initial frequency band, and compensates for distortion caused bycomponents of the forward path. The negative feedback action willgenerally reduce both distortion caused by non-linear steady stateimpedance of the switches as well as distortion resulting from switchingglitches. Preferably, the level-discretization modulator used by thepower amplifier is also included in the forward path of the loop so thatthe feedback path compensates for distortion, such as quantizationnoise, caused by the modulator.

[0014] By performing frequency up-conversion in the forward path of thepower amplifier after the level-discretization modulator, therequirements on the level-discretization modulator with regard to themodulator input signal frequency will be quite relaxed, making itpossible to operate the modulator on low or intermediate frequencies.

[0015] The invention thus solves the combined problem of enhancinglinearity and preserving high efficiency in power amplifiers withintegrated frequency up-conversion.

[0016] Advantageously, the local oscillator clock frequency used forfrequency up-conversion and down-conversion is an integer multiple ofthe modulator clock frequency.

[0017] For efficient handling of image frequency bands, the feedbackpath preferably comprises at least one wide-band filter, which iscentered around the local oscillator clock frequency and with abandwidth including both lower and upper information side bands.

[0018] A particular advantage of the invention is that it is possible toallow more non-linearity in the switch stage, since the negativefeedback enables suppression of the distortion caused by non-linearitiesin the switches. Thus, somewhat slower switches can be used, which meansthat less expensive switch technologies are available.

[0019] Another related aspect of the invention concerns animplementation using a digital modulator. In such a case, the feedbacksignal has to be digitized in an A/D-converter. In order to handle thevery short-time, high signal-level glitches caused by non-idealswitching, a wide-band filter is introduced before the A/D-converter inthe feedback path for averaging the switching glitches and thus reducingthe required sampling frequency and the required dynamic range of theA/D-converter. By using a wide-band filter, a minimum of delay isintroduced into the feedback path and hence the stability of the overallsystem is maintained.

[0020] In order to reduce the required dynamic range of theA/D-converter even further, only the error signal representative of thenon-idealities of the switch stage is preferably transferred in theanalog domain and digitized by the A/D-converter. Hence, most of thefeedback goes directly in the digital domain.

[0021] The highly linear power amplifier of the invention isparticularly suitable for use in RF applications with multi-channel ormulti-level signals.

[0022] The invention offers the following advantages:

[0023] Enhanced linearity,

[0024] Maintained stability and amplifier efficiency;

[0025] The level-discretization modulator can operate in a reasonablefrequency band;

[0026] Reasonable requirements on the A/D-converter required for animplementation using a digital modulator, and

[0027] Reduced requirements on switch speed, making less expensiveswitch technologies available.

[0028] Other advantages offered by the present invention will beappreciated upon reading of the below description of the embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The invention, together with further objects and advantagesthereof, will be best understood by reference to the followingdescription taken together with the accompanying drawings, in which:

[0030]FIG. 1 is a schematic high-level block diagram of a radiotransmitter based on a power amplifier,

[0031]FIG. 2 is a schematic block diagram of a linearized poweramplifier;

[0032]FIG. 3 is a schematic block diagram of a power amplifier accordingto a preferred embodiment of the invention;

[0033]FIG. 4 is a schematic block diagram of a power amplifier accordingto another preferred embodiment of the invention;

[0034]FIG. 5 is a schematic block diagram of a power amplifier accordingto yet another preferred embodiment the invention;

[0035]FIG. 6 illustrates the possibility to include a part of the outputfiltering stage in the error compensation loop of a power amplifieraccording to the invention; and

[0036]FIG. 7 is a schematic block diagram of a very stable poweramplifier implemented without frequency up-conversion.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

[0037] Power amplifiers can be found in various applications in manyfields of technology such as consumer electronics, radar technology andradio communication. In the following, the invention will be describedwith reference to a particular application within the field of radiocommunication. It should though be understood that the invention is notlimited thereto, and that other applications are feasible as well.

[0038] In a typical radio application, as schematically illustrated inthe high-level block diagram of FIG. 1, a power amplifier is arranged ina radio transmitter for simultaneous amplification of severalnarrow-band channels. In a very basic realization, the transmitter 10comprises a general input unit 12 for combining the input signals into acomplex multi-channel signal, a power amplifier 14 for simultaneousamplification of the multiple channels and a transmission element 16.Such a basic realization of course requires that the input signals aremodulated RF signals within the desired target frequency band If theinput signals are base-band signals, up-conversion and filtering arealso required The transmitter illustrated in FIG. 1 is adapted foramplification and transmission of for example several FDMA/TDMA carrierwaves, but can easily be modified for amplification and transmission ofa carrier wave on which several CDMA channels are superimposed, or formulti-level linear modulation formats.

[0039] As outlined in the background section, complex multi-channel ormulti-level signals requires a power amplifier with a high degree oflinearity so that the phases and amplitudes of all the signal componentsare preserved in the amplified output signal. At the same time, a highdegree of amplifier efficiency is required. The state-of-the-artsolution to the combined problem of enhancing linearity and preservinghigh efficiency is represented by switch-based power amplifiers usingsigma-delta modulation in combination with power switching techniques.

[0040] However, a careful analysis by the inventors of the conventionalswitch-based power amplifiers has revealed that the power switches havea non-negligible effect on the overall linearity of the power amplifier.Ideally, at each point in time, one and only one switch should be closedand have zero “on-impedance”, and all the other switches should be openand have infinite “off-impedance”. If these conditions are not fulfilledat some point in time, the result will inevitably be undesired energydissipation and reduced efficiency. However, it has been recognized thatnon-ideal switching not only affects the amplifier efficiency, but alsointroduces distortion that will reduce the overall linearity of thepower amplifier. The problem can be divided into two parts withdifferent causes. The first is the possible non-zero impedance of theswitch in its closed steady state. While a positive on-impedance willalways reduce the amplifier efficiency, a non-linear steady-stateon-impedance will also introduce distortion. The same applies inprinciple to the steady-state off-impedance for an open switch, but thisis less problematic in practice. The second cause relates to thetransition speed of the switch between closed and open states. A finitetransition speed of the switches generally results in switchingtransients, also referred to as switching glitches, that will reduce theefficiency but also introduce distortion.

[0041] A necessary requirement for minimizing switching glitches is thatthe switches are extremely fast compared to the RF period and extremelywell synchronized. This severely limits the maximum frequency at which apractical and efficient implementation can be built by today'scomponents. In practical cases, the switch speed required for meetingthe usually very strict linearity requirements is much greater than thespeed required for making the amplifier sufficiently efficient Thismeans that the requirement on switch speed can be significantly reducedif another form of linearization is devised.

[0042] Linearization may be achieved by including the switch stage inthe forward path of an error compensation loop, allowing thenon-linearities of the switch stage to be compensated for by thefeedback action of the loop. This solution reduces both the distortioncaused by a non-linear steady-state impedance and distortion caused byswitching transients.

[0043] A first attempt to solve the linearization problem might be toprobe the final output of the amplifier and subtract a version of thissignal from the input signal. A problem with this scheme is that theband-pass filter normally included as a final output stage has too largetime-delay, since its bandwidth is about as narrow as the noise-shapingbandwidth of the level-discretization modulator. This means that thetotal feedback system may become unstable, or that very limiteddistortion reduction can be expected from the system.

[0044] At first thought, the bandwidth of the feedback signal should belimited to the signal bandwidth. In that case, the modulator knows whatsignal is present at the output in the whole relevant frequency rangeand should be able to adjust the input signal of the quantizer so thatthe output signal from the power switches is adjusted towards thedesired output. However, if the time-delay in the feedback path isnon-negligible, the system may become unstable or the performance willbe inadequate. Signal theoretical considerations show that thetime-delay of a filter is larger than or equal to the order of theinverse of the bandwidth. Thus, it is normally required that any filterin the feedback path actually has a bandwidth that is much larger thanthe signal bandwidth in order not to introduce too large time-delay.

[0045] Therefore, it is better to sense the output signal of the switchstage before the output filter stage and feed the sensed signal back tothe input of the power amplifier, as illustrated in FIG. 2.

[0046]FIG. 2 is a schematic block diagram of a linearized poweramplifier. The power amplifier 100 comprises a level-discretizationmodulator 110, a switch-based amplifying stage 120, an output filteringstage 130, a feedback probe 140, an optional filter 150 as well as anadder 160. The overall error compensation loop is defined by a forwardpath, which basically comprises the level-discretization modulator 110and the switch-based amplifying stage 120, and a negative feedback pathfor enabling suppression of distortion caused by the modulator 110 andthe switch-based amplifying stage 120.

[0047] In this particular embodiment, the level-discretization modulator110 is an analog modulator that converts a continuous input signal intoa digital signal having a finite number of discrete levels. Themodulator 110 comprises a filter 112 with a response function G(s), anda quantizer 114. The quantizer 114 in the level-discretization modulator110 converts the filtered signal into a digital signal having M possiblediscrete levels at points of time given by an internal clock (notshown). It should though be understood that the invention is not limitedto this particular type of modulator, and that any level-discretizationmodulator can be used by the invention.

[0048] The switch-based amplifying stage 120 is responsible for theactual amplification and comprises a decoder/driver 122 in associationwith a number, M, of power switches S1, S2, . . . , SM that areconnected to respective voltage sources U1, U2, . . . , UM. The digitaloutput signal of the level-discretization modulator 110 controls thepower switches via the decoder/driver 122. The voltages U1, U2, . . . ,UM are proportional to the levels of the quantizer 114. Ideally, at agiven time, one and only one switch is closed in accordance with thesignal level of the quantizer output in order to connect the appropriatevoltage to the output of the switch stage and thus provide proper poweramplification. For example, if the quantizer 114 of thelevel-discretization modulator 110 outputs the discrete level v_(n),then switch S_(n) is closed and all the other switches are open. Thismeans that the voltage at the output of the switch-based amplifyingstage 120 will be equal to U_(n). Although the power switches have beenillustrated as connected to voltage sources, the voltage sources can bereplaced by current sources with modifications that are obvious to theskilled person.

[0049] For additional information on switch-based power amplification,reference is made to the International Patent Application WO 98/11683and the corresponding U.S. Pat. No. 6,094,458.

[0050] The output signal of the switch-based amplifying stage 120, whichis also the output of the forward path of the error compensation loop,is sensed by the probe 140 and fed back to the adder 160 at the input ofthe power amplifier, thus completing the loop. In order to provide thenecessary feedback action, the adder 160 subtracts the feedback signalfrom the input signal. Any distortion within the pass-band of theoverall loop filter, taking the response of the involved filters andcomponents including the linearized response of the quantizer 114 intoaccount, will be reduced by the feedback action of the errorcompensation loop.

[0051] Since the output signal of the switch-based amplifying stage 120acts as input to the output filtering stage 130 of the power amplifier,it is important that the feedback probe 140 is capable of sensing theoutput voltage from the power switches without causing any significantreduction of the signal power or introducing any disturbances in theoutput voltage. For this reason, the probe 140 is generally realized asa high-impedance probe. The filtering stage 130 is generally in form ofa band-pass filter (BPF) with a pass-band that preferably includes onlyfrequencies that are well within the pass-band of the overall loopfilter.

[0052] However, it may be difficult or even practically unfeasible tooperate the level-discretization modulator on high input signalfrequencies, such as high radio frequencies (HRF). In order to avoidthis problem, frequency up-conversion to the radio frequency band may beintegrated into the power amplifier after the level-discretizationmodulator so that the modulator is allowed to operate in a comfortablefrequency range, typically in the baseband or intermediate frequencyband. The design of the feedback path in this type of power amplifierwith integrated frequency up-conversion however needs some particularattention, requiring a frequency down-conversion mixer in the feedbackpath for down-conversion to the initial frequency band.

[0053] In the following, various preferred embodiments of a poweramplifier with integrated frequency up-conversion and a properlydesigned linearizing feedback will be described with reference to FIGS.3-6.

[0054]FIG. 3 is a schematic block diagram of a preferred embodiment of apower amplifier according to the present invention, using frequencyup-conversion to the RF domain. The power amplifier 200 is based on alevel-discretization modulator 210 and a mixing and amplifying stage 225formed by an up-conversion mixer 270 in combination with a switch-basedamplifying stage 220. The overall error compensation loop is defined bya forward path, which basically comprises the level-discretizationmodulator 210 and the mixing and amplifying stage 225, and a negativefeedback path for enabling suppression of distortion caused by themodulator 210 as well as components in the mixing and amplifying stage225. The negative feedback path now comprises a down-conversion mixer280 for down-conversion to the initial frequency band of the modulatorinput signals. The up-conversion mixer 270 and the down-conversion mixer280 are controlled by a local oscillator clock, which is preferablyproduced by a frequency multiplier 287 fed by the main oscillator 285 ofthe power amplifier circuit. The main oscillator 285 also distributesthe fundamental clock to the quantizer 214 of the modulator 210.Naturally, the clock frequency for the analog level-discretizationmodulator 210 is lower than for realizations without up-conversion. Ifthe local oscillator clock frequency is an integer multiple of thefundamental clock frequency of the main oscillator, the local oscillatorsignal can be a simple two-level signal without any distortion beingintroduced. The quantizer generally produces a lot of extra signalreplica in different frequency regions of the quantizer output. Theadvantage of using the local oscillator clock frequency as an integermultiple (×N) of the fundamental clock frequency, is that in-mixing ofthe replica by harmonic and image mixing in the mixer will occur in sucha way that the final output signal is not distorted. This holds trueboth for up-conversion and down-conversion, since the local oscillatorsignal is the same for both mixers.

[0055] It is possible to use a non-integer frequency ratio, but thenspecial care has to be taken in order not to distort the final outputsignal.

[0056] In order to protect the down-conversion mixer from high-levelsignals such as switching glitches caused by the switch-based amplifyingstage 220, a wide-band filter 290 is preferably introduced in thefeedback path. It is important to note the linearity of the overallsystem is dependent on good linearity of the down-conversion mixer,since distortion caused by the down-conversion mixer is not compensatedfor by the feedback. The introduction of the wide-band filter 290 helpsmaintaining the linearity by reducing the bandwidth and dynamic range ofthe input signal to the down-conversion mixer 280. In order to maintainthe stability of the overall feedback system, it is important that thewide-band filter 290 has a relatively small time-delay.

[0057] For an implementation with up-conversion it is worthwhile todiscuss the image frequency bands. If a band-pass modulator is used, theoutput filtering stage of the power amplifier will easily filter out theimage frequency bands. Also, the wide-band filter or filters provided inthe negative feedback path of the error compensation loop will assist infiltering these frequency bands out. In fact, the image frequency bandsare generally not a problem if the information signal is rather close tothe center of a particular image (Nyquist zone). The situation ishowever different if the information signal band is closer to the edgeof the Nyquist zone (closer to an integer multiple of half the modulatorclock frequency). Consider as an example a low-pass modulator. Theoutput filtering stage now has to be very narrow-banded to filter awaythe image bands. The wide-band filter in the overall feedback pathshould then preferably be centered around the local oscillator frequency(supposed to be an integer multiple of the modulator clock frequency),and at least of such a wide bandwidth that it includes both the fulllower and the full upper information side bands. The down-convertedsignal will then be correct.

[0058] Alternatively, the image problem can be handled by cancellationof the non-wanted side-bands using quadrature input signals to twoparallel modulating and switching units, and combining the outputsignals of the parallel units before or after the output filteringstage. A slightly different variant is not directed at canceling theside bands, but rather to use them as two linearly independent channelsin a traditional “I and Q” (in-phase and quadrature-phase) approach. Forthe skilled person a lot of different implementations of the linearizedpower amplifier are then immediately obvious.

[0059] The basic realization of FIG. 3 gives a practically workinglinearized power amplifier if an analog level-discretization modulatoris used. However, for an implementation using a digital modulator, thefeedback signal has to be digitized in an analog-to-digital converter(A/D-converter). The non-ideal switching of the switch stage oftencreates very short time, high signal-level glitches. In order todigitize the switching glitches, an A/D-converter with extremely highsampling frequency and very high dynamic range is generally required.The wide-band filter introduced in the feedback path normally handlesthis problem.

[0060]FIG. 4 is a schematic block diagram of a power amplifier accordingto another preferred embodiment of the invention. The power amplifier ofFIG. 4 is similar to that of FIG. 3, except for an A/D-converter in thefeedback path, and the level-discretization modulator being digitalrather than analog. The clock signal distribution is only included inFIG. 4 to schematically illustrate which components that are using thefundamental clock frequency and which components that are using thelocal oscillator clock frequency. In practice, standard engineeringpractice has to be used to tailor the clock signal to each component,for example to minimize time-delay and at the same time eliminate therisk for set-up time violations and hold time violations. The digitallevel-discretization modulator 310 is generally operable for convertinga relatively large number of discrete levels to a smaller number ofdiscrete levels. Compared to an analog modulator, the main difference isthat the filter 312 (response function G(z)) of the digital modulator310 is clocked by an internal clock, while the quantizer 314 is not. Theuse of a digital modulator means that the signal sensed at the output ofthe switch-based amplifying stage 320 now has to go through anA/D-converter 395 provided in the feedback path.

[0061] In order to reduce the required bandwidth and dynamic range ofthe A/D-converter 395, a wide-band filter 390 is placed in the inputpath of the A/D-converter, preferably before the down-conversion mixer380, for removing high-frequency components of the switching glitches.Accordingly, the glitches are averaged out and the required dynamicrange of the A/D-converter 395 is significantly reduced, since theconverter now does not have to resolve the full glitch. Furthermore, theuse of the wide-band filter 390 also reduces the required sampling rateof the A/D-converter. For instance, if the bandwidth of the filter 390is half the clock frequency of the level-discretization modulator 310,the A/D-converter 395 can sample at that same frequency. In order tomaintain the stability of the overall feedback system, the A/D-converter395 as well as the wide-band filter 390 should have small time-delays.In this respect, a suitable type of converter to be used in the feedbackpath may be a so-called fast flash A/D-converter. The filter 390 may bea low-pass or band-pass filter with a relatively large bandwidth, makingthe corresponding time-delay rather small.

[0062] Although the embodiment of FIG. 4 provides a practicalrealization when a digital modulator is used, the demands on theA/D-converter are still relatively tough. In general, the dynamic rangeof the A/D-converter has to be larger than that of the informationsignal.

[0063] A further reduction of the required dynamic range of theA/D-converter may be obtained by extracting the error signal thatrepresents the distortion of the switch-based amplifying stage from theoutput signal of the switch-based amplifying stage and digitizing onlythe error signal in the A/D-converter. This significantly reduces therequired dynamic range of the A/D-converter, allowing a rather lowdynamic range A/D-converter to be used and making wide-band and smalllatency designs feasible. A practical realization of such a reduction ofthe dynamic range will now be described with reference to FIG. 5.

[0064]FIG. 5 is a schematic block diagram of yet another preferredembodiment of the invention. In this realization, thelevel-discretization modulator 410 of the power amplifier 400 ispreferably a digital sigma-delta modulator, with an internal feedbackpath entirely in the digital domain for suppression of quantizationnoise. The sigma-delta modulator 410 basically comprises a filter 412with response function G(z), a quantizer 414, and an internal feedbackpath. The internal feedback path of the modulator preferably has afilter 418 with response function F(z). Furthermore, the power amplifier400 is configured with an arrangement for extracting an error signal,which represents the distortion introduced by the power switches, fromthe output signal of the switch-based amplifying stage 420. Thisarrangement basically comprises a digital-to-analog converter(D/A-converter) 492 for converting the output signal of the sigma-deltamodulator 410 into the analog domain, a second wide-band filter 493 forfiltering the output of the D/A-converter 492 and an adder/subtractor494 for subtracting the output of the second filter 493 from the outputof the down-conversion mixer 480, thus extracting the error signal. Theerror signal is then converted in an A/D-converter 495 of relatively lowdynamic range and added into the internal feedback loop of thesigma-delta modulator 410 by means of a digital adder 419. The wide-bandfilters 490 and 492 should be matched as closely as possible in order tosubtract the signal part that is not distorted by the switching to aslarge an extent as possible prior to the A/D-conversion. As can be seenfrom FIG. 5, most of the feedback goes entirely in the digital domain inthe internal feedback path of the sigma-delta modulator 410, whereasonly the error signal of the switch stage goes in the analog domain.

[0065]FIG. 6 illustrates the possibility to include a part of the outputfiltering stage in the error compensation loop of the power amplifier ofthe invention. The power amplifier of FIG. 6 is identical to that ofFIG. 5 except for the filtering stage 530 and the sigma-delta filters515 and 517. In this particular embodiment, the output filtering stage530 comprises two separate units 532, 534. The first filtering unit 532,which has a relatively wide bandwidth, is included in the errorcompensation loop, while the second filtering unit 534 is arrangedoutside the loop. The inclusion of the filtering unit 532 in the errorcompensation loop is compensated for by modifying the filter 418 (F(z))of FIG. 5, splitting that filter into two different parallel filters 515(F1(z)) and 517 (F2(z)) that are arranged for receiving the signals fromthe quantizer 514 and the A/D-converter 595, respectively. Preferably,the wide-band filter 593 (low-pass or band-pass) is matched to thecascade connection of the filtering unit 532 and the wide-band filter590 (low-pass or band-pass).

[0066] Naturally, the inclusion of at least part of the output filteringstage in the error compensation loop is applicable also for a poweramplifier with an analog level-discretization modulator.

[0067] It should be understood that the invention is not limited to themixing and amplifying stage illustrated in FIGS. 3-6. In an alternativeimplementation, the up-conversion takes place “in” the power switches.The local oscillator signal is then introduced in the high-power path byreplacing the constant voltages U1, U2, . . . UM with alternatingvoltage sources (or current sources). The frequency up-conversion in theforward path may thus take place before the amplification or togetherwith the amplification, or possibly even after the amplification.

[0068] If a highly linear level-discretization modulator is used, it maybe sufficient to provide the mixing and amplifying stage, or theswitch-based amplifying stage alone, with an internal feedback path fromthe output to the input of the stage to enable suppression of distortioncaused by non-linearities of the switches.

[0069] The measures for reducing the required dynamic range of theA/D-converter when using a digital modulator can be used independentlyof frequency up-conversion in the forward path and down-conversion inthe feedback path, as schematically illustrated in FIG. 7. The poweramplifier of FIG. 7 is based on a digital modulator 610 such as asigma-delta modulator, in series with a switch-based amplifying stage620, an output filtering stage 630. The feedback path basicallycomprises a feedback probe 640, a wide-band filter 690, and anA/D-converter 695. The wide-band filter 690 introduced before theA/D-converter in the feedback path handles the very short-time, highsignal-level glitches caused by non-ideal switching by averaging theswitching glitches, thus reducing the required sampling frequency andthe required dynamic range of the A/D-converter. By using a wide-bandfilter, a minimum of delay is introduced into the feedback path andhence the stability of the overall system is maintained.

[0070] In order to reduce the required dynamic range of theA/D-converter 695 even further, only the error signal representative ofthe non-idealities of the switch stage is preferably transferred in theanalog domain and digitized by the A/D-converter. The power amplifier600 is therefore configured with an arrangement for extracting the errorsignal, which represents the distortion introduced by the powerswitches, from the output signal of the switch-based amplifying stage620. This arrangement basically comprises a digital-to-analog converter(D/A-converter) 692 for converting the output signal of the sigma-deltamodulator 610 into the analog domain, a second wide-band filter 693 forfiltering the output of the D/A-converter 692 and an adder/subtractor694 for subtracting the output of the second filter 693 from the outputof the first wide-band filter 690, thus extracting the error signal. Theerror signal is then converted in the A/D-converter 695, which may be ofrelatively low dynamic range, and added into the internal feedback loopof the sigma-delta modulator 610 by means of a digital adder 619. Hence,most of the feedback goes directly in the digital domain. The wide-bandfilters 690 and 692 are preferably matched.

[0071] It is normally recommendable to use a band-pass filter ofrelatively wide bandwidth in the feedback path when the output signal isup-converted to radio frequency, and to use a wide-band low-pass filterwhen the output signal is not up-converted. It should however beunderstood that the boundary between low-pass and band-pass filters isquite vague, and that the skilled person has to adapt the filterproperties of the so-called wide-band filters according to theparticular application.

[0072] The embodiments described above are merely given as examples, andit should be understood that the present invention is not limitedthereto. Further modifications, changes and improvements which retainthe basic underlying principles disclosed and claimed herein are withinthe scope and spirit of the invention.

What is claimed is:
 1. A power amplifier based on a level-discretizationmodulator in series with a mixing and amplifying stage for frequencyup-conversion to radio frequency and for switch-based amplification,wherein said power amplifier further includes an error compensation loopcomprising: a forward path including at least said mixing and amplifyingstage; and a negative feedback path for enabling suppression ofdistortion caused by components of said forward path, said negativefeedback path including a frequency down-conversion mixer fordown-conversion to the initial frequency band.
 2. The power amplifieraccording to claim 1, wherein the local oscillator clock frequency usedfor up-conversion and down-conversion is an integer multiple of themodulator clock frequency.
 3. The power amplifier according to claim 1,wherein said feedback path further comprises at least one wide-bandfilter, said wide-band filter being centered around the local oscillatorclock frequency and having a bandwidth including both lower and upperinformation side bands.
 4. The power amplifier according to claim 1,wherein said level-discretization modulator is digital, said forwardpath further comprises said digital modulator and said feedback pathfurther comprises an A/D-converter and a first wide-band filter arrangedbefore said A/D-converter for filtering the output signal of saidforward path in order to average switching glitches from saidswitch-based amplifying stage.
 5. The power amplifier according to claim4, further comprising means for extracting an error signal representingthe distortion of said switch-based amplifying stage from the outputsignal of said forward path, and wherein said A/D-converter is arrangedfor digitizing said error signal.
 6. The power amplifier according toclaim 5, wherein said level-discretization modulator is a sigma-deltamodulator having an internal feedback loop, and said feedback pathfurther comprises means for adding said digitized error signal into theinternal feedback loop of said sigma-delta modulator.
 7. The poweramplifier according to claim 6, wherein said means for extracting anerror signal comprises: said first wide-band filter provided in saidfeedback path for filtering the output signal of said forward path; aD/A-converter for converting the output signal of said modulator intoanalog form; a second wide-band filter, matching said first wide-bandfilter, for filtering the analog sigma-delta output signal; and asubtractor for subtracting the filtered output signal of said secondwide-band filter from the filtered output signal of said first wide-bandfilter to generate said error signal.
 8. The power amplifier accordingto claim 4, wherein said first wide-band filter has a bandwidth largerthan the bandwidth of the input signal to said power amplifier.
 9. Thepower amplifier according to claim 1, wherein said forward path furthercomprises at least part of a filtering stage connected after saidswitch-based amplifying stage.
 10. The power amplifier according toclaim 9, wherein said filtering stage is divided into a number offiltering units, and said forward path comprises at least one of saidfiltering units.
 11. The power amplifier according to claim 1, whereinsaid feedback path comprises a high-impedance probe for extracting theoutput signal of said forward path.
 12. The power amplifier according toclaim 1, wherein said mixing and amplifying stage includes a frequencyup-conversion mixer and a switch-based amplifying stage.
 13. The poweramplifier according to claim 12, wherein said switch-based amplifyingstage comprises a decoder in connection with a plurality of powerswitches.
 14. A power amplifier based on a level-discretizationmodulator in series with a mixing and amplifying stage for frequencyup-conversion to radio frequency and for switch-based amplification,wherein said power amplifier includes an error compensation loopcomprising: a forward path including said level-discretization modulatorand said mixing and amplifying stage; and a negative feedback path forenabling suppression of distortion caused by said level-discretizationmodulator and said mixing and amplifying stage, said negative feedbackpath including a frequency down-conversion mixer for down-conversion tothe initial frequency band.
 15. A transmitter having a power amplifierthat comprises a level-discretization modulator in series with a mixingand amplifying stage for frequency up-conversion to radio frequency andfor switch-based amplification, wherein said power amplifier includes aloop comprising: a forward path including at least said mixing andamplifying stage; and a negative feedback path for enabling suppressionof distortion caused by components of said forward path, said negativefeedback path including a frequency down-conversion mixer fordown-conversion to the initial frequency band.
 16. The transmitteraccording to claim 15, wherein the local oscillator clock frequency usedfor up-conversion and down-conversion is an integer multiple of themodulator clock frequency.
 17. The transmitter according to claim 15,wherein said feedback path further comprises at least one wide-bandfilter, said wide-band filter being centered around the local oscillatorclock frequency and having a bandwidth including both lower and upperinformation side bands.
 18. The transmitter according to claim 15,wherein said level-discretization modulator is digital, said forwardpath further comprises said digital modulator and said feedback pathfurther comprises an A/D-converter and a first wide-band filter arrangedbefore said A/D-converter for filtering the output signal of saidforward path in order to average switching glitches from saidswitch-based amplifying stage.
 19. The transmitter according to claim18, wherein said power amplifier further comprises means for extractingan error signal representing the distortion of said switch-basedamplifying stage from the output signal of said forward path, and saidA/D-converter is arranged for digitizing said error signal.
 20. Thetransmitter according to claim 19, wherein said level-discretizationmodulator is a sigma-delta modulator having an internal feedback loop,and said feedback path further comprises means for adding said digitizederror signal into the internal feedback loop of said sigma-deltamodulator.
 21. The transmitter according to claim 20, wherein said meansfor extracting an error signal comprises: said first wide-band filterprovided in said feedback path for filtering the output signal of saidforward path; a D/A-converter for converting the output signal of saidmodulator into analog form; a second wide-band filter, matching saidfirst wide-band filter, for filtering the analog sigma-delta outputsignal; and a subtractor for subtracting the filtered output signal ofsaid second wide-band filter from the filtered output signal of saidfirst wide-band filter to generate said error signal.
 22. Thetransmitter according to claim 18, wherein said first wide-band filterhas a bandwidth larger than the bandwidth of the input signal to saidpower amplifier.
 23. The transmitter according to claim 15, wherein saidforward path further comprises at least part of a filtering stageconnected after said switch-based amplifying stage.
 24. The transmitteraccording to claim 23, wherein said filtering stage is divided into anumber of filtering units, and said forward path comprises at least oneof said filtering units.
 25. The transmitter according to claim 15,wherein said feedback path comprises a high-impedance probe forextracting the output signal of said forward path.
 26. The transmitteraccording to claim 15, wherein said mixing and amplifying stage includesa frequency up conversion mixer and a switch-based amplifying stage. 27.The transmitter according to claim 26, wherein said switch-basedamplifying stage comprises a decoder in connection with a plurality ofpower switches.
 28. A power amplifier based on a digitallevel-discretization modulator in series with a switch-based amplifyingstage, wherein said power amplifier further includes an errorcompensation loop comprising: a forward path including saidlevel-discretization modulator and said switch-based amplifying stage;and a negative feedback path for enabling suppression of distortioncaused by said level-discretization modulator and said switch-basedamplifying stage, said feedback path including an A/D-converter and afirst wide-band filter arranged before said A/D-converter for filteringthe output signal of said forward path in order to average switchingglitches from said switch-based amplifying stage.
 29. The poweramplifier according to claim 28, wherein said power amplifier furthercomprises means for extracting an error signal representing thedistortion of said switch-based amplifying stage from the output signalof said forward path, and said A/D-converter is arranged for digitizingsaid error signal.
 30. The power amplifier according to claim 29,wherein said digital level-discretization modulator is a sigma-deltamodulator having an internal feedback loop, and said feedback pathfurther comprises means for adding said digitized error signal into theinternal feedback loop of said sigma-delta modulator.